1. Field of the Invention
The present invention relates to an optical information recording medium, especially an optical disk, for recording and reproducing a large capacity of information with a laser beam, and recording and reproduction therefor.
2. Description of Prior Art
A read-only optical recording medium with a laser beam includes optical disks called as a compact disk (CD), a laser disk (LD) and a digital video disk (DVD). Among the commercial read-only optical information recording media, a DVD disk can record signals at the highest density. According to the DVD format, a read-only DVD has diameter of 120 mm, and a user capacity of 4.7 GB at maximum for single layer type for single side reading, that of 9.4 GB for single layer type for double side reading, or that of 8.5 GB for two-layers type for single side reading. For example, a two-layers type DVD for single side reading has first and second information layers, and signals recorded in either of first and second information layers can be reproduced with a laser beam irradiated from one side of the optical disk. A method for fabricating a read-only DVD having a multi-layer structure is described for example in U.S. Pat. No. 5,126,996.
Optical information recording media which can record and reproduce signals with a laser beam include a phase-change type optical disk, a magneto-optical disk and a dye disk. In a phase-change type optical disk, a chalcogenide is used usually as a recording thin film material. Generally, when the recording thin film material is in the crystalline state, it is taken as a non-recorded state. A signal is recorded by irradiating a laser beam to melt the magnetic thin film material and to cool it quickly to the amorphous state. When a recorded signal is erased, a laser beam having a power lower than that for recording is irradiated to change the recording layer to the crystalline state. Because the chalcogenide magnetic thin film is formed with deposition in the amorphous state, it is necessary to crystallize all the surface of the recording region beforehand to result in the non-recorded state. This initial crystallization is usually included in a part of the disk manufacturing steps. A recording thin film is crystallized with a light source of a laser light source or a flash one.
In order to enhance the signal recording speed for a phase-change type optical disk on which recording and erasing are possible, so-called light absorption correction is proposed which is suitable for high linear speed recording (for example, Japanese Patent laid open Publication 5-298747/1993, Japanese Patent Publication 8-1707/1996, Japanese Patent laid open Publications 7-78354/1995 and 7-262612/1995). In all the proposals, the light absorptance of the recording thin film for the laser beam for recording is larger in the crystalline state than in the amorphous state of the recording thin film.
For an ordinary phase-change type optical disk without the optical absorption correction, the absorptance of the recording thin film for a laser beam for recording is smaller in the crystalline state than in the amorphous state. For such an optical disk, it is known that the temperature of the recording thin film attained on melting with the laser beam becomes higher in an area which was amorphous before the laser irradiation than in an area which was crystalline before the laser irradiation. This is ascribed to that latent heat is not needed to be supplied on melting from a starting point of the crystalline state but not so from a starting point of the amorphous state. It is also ascribed to that the amorphous state can be heated efficiently to a higher temperature because the thermal conductivity is lower in the amorphous state than in the crystalline state. Further, for an optical disk without the optical absorption correction, a large difference on the temperature attained on melting arises especially on high linear velocity recording. The reason is that a temperature rise effect other than the direct temperature rise due to laser irradiation, that is, temperature rise due to heat diffusion around the laser irradiation point, is smaller than the counterpart in low linear velocity recording. If the temperature attained on melting is changed according as the recording thin film was amorphous or crystalline before laser irradiation for recording, or according as it is a recorded mark or a non-recorded region, the shape of a recorded mark formed newly by overwriting is distorted, and good reproduction jitter cannot be obtained. The optical absorption correction is proposed for realizing good overwrite characteristic on high linear velocity recording by canceling the above-mentioned difference in temperature rise profile depending on whether the recording thin film is amorphous or crystalline.
Further, in order to increase the recording density of an optical disk on which recording is possible or an optical disk on which both recording and erasing are possible, so-called land and groove recording is proposed, wherein signals are recorded on grooves formed on the substrate surface and on lands between the grooves (for example, Japanese Patent laid open Publication 5-282705/1993).
Further, in order to increase the recording capacity of an optical disk on which both recording and erasing are possible, a two-layers structure is already proposed wherein information is read from one side thereof (for example, Japanese Patent laid open Publication 9-212917/1997). The optical disk of two-layers structure has two phase-change recording films, and information is read or erased in each of the recording films by irradiating a laser beam from one side of the optical disk. The recording capacity is increased twice by using two recording films. As to the initialization of an optical disk of the two-layers structure, it is proposed to shorten the initialization time by irradiating the two recording layers at the same time (for example, Japanese Patent laid open Publication 9-91700/1997).
Though the idea of the two-layers structure of an optical disk on which both recording and erasing are possible is already proposed (for example, Japanese Patent laid open Publication 9-212917/1997) in order to enhance the recording capacity, it has not yet been used practically because following problems are not solved. Hereinafter, xe2x80x9cfirst information layerxe2x80x9d denotes a recordable layer positioned at the front side viewed from the incident laser beam for recording and reproduction, and xe2x80x9csecond information layerxe2x80x9d denotes a recordable layer positioned at the back side viewed from the incident laser beam.
1) A structure of the first information layer is not known, that has high transmittance, and high recording sensitivity at a laser beam wavelength for recording, erasing and reproducing signals, that enables land and groove recording, and that has a good repetition characteristic of recording and erasing.
2) A structure of the second information layer is not known, that has high recording sensitivity and sufficiently high reflectance at non-recorded portions at a laser beam wavelength for recording, erasing and reproducing signals, that enables land and groove recording, and that has a good repetition characteristic of recording and erasing.
3) A structure of the two-layers optical disk is not known that enables high speed overwriting to the first and second information layers.
4) In the initial crystallization of the optical recording medium having two layers of phase-change recording thin films, the sensitivity of the initial crystallization is different among the first and second information layers, and it is necessary to perform the initialization at different initialization conditions. Though the initialization with focus servo for each information layer is proposed (for example, Japanese Patent laid open Publication 9-91700/1997), this needs an optical system having the focal depth narrower than the thickness of the separation layer. Therefore, this is a problem for an apparatus for high speed initialization by laser irradiation over a large area.
5) The transmittance of the first information layer is different between when signals are recorded and when signals are not recorded. Therefore, the amplitude of reproduced signals for the second information layer is different between the two cases. This causes read errors for the second information layer. Further, the recording sensitivity is different between the two cases, and an optimum recording power for the second information layer cannot be determined.
6) For a recordable optical information recording medium with a multi-layer structure, the transmittance of the first information layer has to be high in order to reproduce signals from the second information layer. Recording for the first information layer is not possible if the absorptance of the first information layer is not high to a certain extent. Then, the reflectance of the first information layer becomes low necessarily. Then, it becomes difficult to reproduce signals recorded in the first information layer well.
7) Because the signals recorded in the second information layer is reproduced with a light which transmits the first information layer which absorbs light twice, the reproduction signals are very small. Therefore, it is difficult to reproduce signals recorded in the second information layer.
A main object of the invention is to provide an optical information recording medium with a multi-layer structure, a recording or reproduction method therefor, and an optical information reproduction apparatus which solves the above problems.
In one aspect of the present invention, an optical information recording medium comprises a first information layer including a first recording layer to which information can be recorded by causing reversible phase change between crystalline and amorphous states by exposure to a light beam, and a second information layer including a second recording layer to which information can be recorded by causing reversible phase change between crystalline and amorphous states by exposure to the light beam. As to the two information layers, a relationship that R1a less than R1c and that R2a greater than R2c holds between reflectance Rc of the first information layer when the first recording layer is in the crystalline state, reflectance R1a of the first information layer when the first recording layer is in the amorphous state, reflectance R2c of the second information layer when the second recording layer is in the crystalline state, and reflectance R2a of the second information layer when the second recording layer is in the amorphous state. Preferably, in the optical information recording medium, a relationship that A1a less than A1c holds between absorptance A1c of the first information layer when the first recording layer is in the crystalline state, and absorptance A1a of the first information layer when the first recording layer is in the amorphous state. Similarly, in the optical information recording medium, a relationship that A2a less than A2c holds preferably between absorptance A2c of the second information layer when the second recording layer is in the crystalline state, and absorptance A2a of the second information layer when the second recording layer is in the amorphous state.
In a second aspect of the invention, an optical information recording medium comprises a first information layer including a first recording layer to which information can be recorded by causing reversible phase change between crystalline and amorphous states by exposure to a light beam, and a second information layer including a second recording layer to which information can be recorded by causing reversible phase change between crystalline and amorphous states by exposure to the light beam. As to the two information layers, a relationship that A1a less than A1c and that A2a less than A2c holds between the absorptances A1c, A1a, A2c and A2a. Preferably, in the optical information recording medium, a relationship that R1a less than R1c holds between the reflectances R1c and R1a. Similarly, in the optical information recording medium, a relationship that R2a greater than R2c holds preferably between the reflectances R2c and R2a. 
In a third aspect of the invention, when an optical information recording medium is fabricated, initial crystallization is performed before the adherence of the two information layers. That is, there are provided first and second information layers including first and second recording layers respectively to which information can be recorded by causing reversible phase change between crystalline and amorphous states by exposure to a light beam. Then, at least one of the first and second information layers is subjected to initial crystallization, and the first and second information layers are adhered.
In a fourth aspect of the present invention, an optical information recording medium comprises a first information layer including a first recording layer to which information can be recorded by exposure to a light beam, an intermediate transparent layer formed on the first information layer, a second information layer including a second recording layer to which information can be recorded by exposure to a light beam, and an overcoat layer formed between the first information layer and the intermediate transparent layer or between the second information layer and the intermediate transparent layer. By providing at least one overcoat layer, the repetition characteristics of recording can be improved.
In a fifth aspect of the invention, an optical information recording medium comprises a first recording layer to which information can be recorded by exposure to a light beam; and a second recording layer to which information can be recorded by exposure to a light beam which transmits the first recording layer. The first recording layer includes a region wherein recorded marks have been formed. The marks are formed so that the first recording layer has about the same transmittance as that in an area where signals are recorded.
In a sixth aspect of the invention, when information is recorded in a second recording layer in a multi-layer optical information recording medium, a reproduction position is set for the recording medium, and it is decided whether signals have been recorded in the first recording layer in front of the reproduction position. Then, signals are recorded to the second recording layer with a laser beam that is transmitted through the first recording layer only when signals are decided to be recorded in the first recording layer in front of the reproduction position.
In a seventh aspect of the invention, when information recorded in first and second layers in an optical information recording medium is reproduced, it is decided whether signals are reproduced from the first recording layer or from the second recording layer. Then, signals recorded in the first recording layer are reproduced with a laser beam of a first power, and signals recorded in the second recording layer are reproduced with a laser beam that transmits through the first recording layer and that has a second power higher than the laser beam of the first power. Thus, an apparatus for reproducing signals from a multi-layer optical information recording medium having the first and second recording layers comprises a rotation driver which supports and drives the multi-layer optical information recording medium, an optical head comprising a light source, which emits a light to the recording medium rotated by the rotation driver, a detector, which detects light reflected from the recording medium, and a controller, which makes the light source emits the light beam having a first power when signals are reproduced from the first recording layer or having a second power higher than the first power when signals are reproduced from the second recording layer.
In an eighth aspect of the invention, when signals are reproduced from a first multi-layer optical information recording medium having two recording layers to which recording and reproduction are performed with a light beam from a side of the recording medium and from a second read-only optical information recording medium, it is discriminated whether the optical information recording medium to be reproduced is the first or second optical information recording medium. Then, signals are reproduced from the second optical information recording medium with a laser beam of a first power when the second optical information recording medium is discriminated, and from the first optical information recording medium with a laser beam of a second power higher than the first power when the first optical information recording medium is discriminated. Thus, an apparatus for reproducing signals from the first and second optical information recording medium comprises a rotation driver, which supports and drives the first or second optical information recording medium having the first or second recording layer, an optical head comprising a light source, which emits a light beam to the first or second optical information recording medium, a detector, which detects light reflected from the first or second optical information recording medium, and a controller, which makes the light source in the optical head emit the light beam having a first power when signals are reproduced from the second optical information recording medium or having a second power higher than the first power when signals are reproduced from the first optical information recording medium.
The above object is solved by the combinations of the characteristics described in the main claims, and the subclaims define further advantageous embodiments of the invention, and this summary of the invention does not necessarily describe all necessary features so that the invention may also be a sub-combination of these described features.
An advantage of the present invention is that an optical information recording medium can record data in two information layers from one side and has a large storage capacity.
Another advantage of the present invention is that signals can be recorded well to, and reproduced from, a multi-layer optical information recording medium.